This invention relates to hydraulic or fluid-actuated drives particularly useful for inserting an element into a pressurized vessel such as for actuating the control rods of a nuclear reactor.
In known types of nuclear power reactors, for example as used in the Dresden Nuclear Power Station near Chicago, Ill., the reactor core comprises a plurality of speed fuel assemblies arranged in an array capable of self-sustained nuclear fission reaction. The core is contained in a pressure vessel wherein it is submerged in a working fluid, such as light water, which serves both as coolant and as a neutron moderator. Each fuel assembly comprises a tubular flow channel, typically of approximately square cross section, surrounding an array of elongated, fuel elements or rods containing suitable fuel material, such as uranium or plutonium oxide, supported between upper and lower tie plates. The fuel assemblies are supported in spaced array in the pressure vessel between an upper core grid and a lower core support plate. The lower tie plate of each fuel assembly is formed with a nose piece which fits in a socket in the core support plate for communication with a pressurized coolant supply chamber. The nose piece is formed with openings through which the pressurized coolant flows upward through the fuel assembly flow channels to remove heat from the fuel elements. A typical fuel assembly of this type is shown, for example, by D. A. Venier et al in U.S. Pat. No. 3,654,077. An example of a fuel element or rod is shown in U.S. Pat. No. 3,378,458.
A plurality of control rods, containing neutron absorbing material, are selectively insertable in the spaces or gaps among the fuel assemblies to control the reactivity of the core. In a known core arrangement, such as shown for example in U.S. Pat. No. 3,020,887, the control rod blades have a cross or cruciform transverse cross section shape whereby the "wings" of the blades of each control rod are insertable in the spaces between an adjacent set of four fuel assemblies.
Suitable control rod drive mechanisms are provided, as shown in the above-mentioned U.S. Pat. No. 3,020,887, to selectively move the control rods into and out of the core whereby the neutron population and hence the core power level can be controlled by the non-fission capture of neutrons by the neutron absorbing material in the control rods. Suitable such neutron absorbing materials, including commonly used boron, are set forth in the above-mentioned U.S. Pat. No. 3,020,887.
During normal reactor operation at power, a significant number of the control rods, for example one-half or more, are withdrawn from the core. The remaining control rods are inserted to various degrees to control reactor power level and shape.
In the event that it becomes necessary to shut down the reaction suddenly, the control rod drives are actuated to insert rapidly all of the control rods to their full extent in the core. (Such an operation commonly is referred to as a "scram.")
Such scram action entails rapid acceleration of the drives and control rods, high speed insertion and, concomitantly, rapid deceleration of the moving masses near the end of the insertion stroke. For example, in known prior systems the scram velocity may be in the order of 5 ft/sec (152 cm/sec). Thus control rod drives typically include a braking or deceleration arrangement, such as a hydraulic buffer, to avoid excessive mechanical shock on the drive mechanism. Examples of such deceleration arrangements are described in U.S. Pat. Nos. 3,020,887 and 3,020,888 which are incorporated herein by reference.
As shown in FIG. 4 of U.S. Pat. No. 3,020,887 the control rod driving device comprises a hollow main or drive piston carrying a hollow indexing tube and fitted for linear motion in a cylinder. Positioned within the drive piston and indexing tube is a stop piston tube at the top end of which is a stop piston. Sealing rings are provided between the drive piston and the stop piston tube and between the drive piston and the cylinder.
A braking arrangement therein includes a series of vertically spaced fluid orifices in the stop piston tube just below the stop piston near the end of drive piston travel. These orifices are progressively closed off by passage thereover and beyond of the sealing rings as the drive piston approaches the end of its stroke. This progressively increases the flow resistance for fluid flow out of the hollow indexing tube with a resulting deceleration force on the drive piston.
A disadvantage of this system is that the deceleration forces create relatively high pressure differences across the sealing rings. More recent requirements for faster scram times require faster acceleration and higher insertion speed (for example, in the order of 10 ft/sec, 305 cm/sec). The result is higher deceleration forces and, consequently, even greater pressure differences across the sealing rings with the danger of premature sealing ring failure.
An object of the invention is a drive braking arrangment which relieves the drive piston sealing rings of deceleration pressures.